In recent years, a variety of optical recording media have been developed and optical pickup devices that carry out recording and reproducing using two alternative types of optical recording media have been known. For example, devices that carry out recording or reproducing with either a DVD (Digital Versatile Disk) or a CD (Compact Disk including CD-ROM, CD-R, CD-RW) have been used. For these two optical recording media, the DVD uses visible light having a wavelength of, for example, approximately 657 nm for improved recording densities while the CD is required to use near-infrared light having a wavelength of approximately 790 nm because there are some recording media that have no sensitivity to visible light. A single optical pickup device, known as a dual-wavelength-type pickup device, uses irradiation light of these two different wavelengths. The two optical recording media described above require different numerical apertures (NA) due to their different features. For example, the DVD is standardized to use a numerical aperture of about 0.60-0.65 and the CD is standardized to use a numerical aperture in the range of 0.45-0.52. Additionally, the thicknesses of the two types of recording disks, including the thicknesses of protective layers or substrates made of polycarbonate (PC), are different. For example, the DVD may have a substrate thickness of 0.6 mm and the CD may have a substrate thickness of 1.2 mm.
As described above, because the substrate thickness of the optical recording medium is standardized and differs according to the type of optical recording medium, the amount of spherical aberration introduced by the substrate is different based on the different standardized thicknesses of the substrates of the different recording media. Consequently, for optimum focus of each of the light beams on the corresponding optical recording medium, it is necessary to optimize the amount of spherical aberration in each light beam at each wavelength for recording and reproducing. This makes it necessary to design the objective lens with different focusing effects according to the light beam and recording medium being used.
Additionally, in response to rapid increases of data capacity required each day, the demand for an increase in the recording capacity of recording media has been strong. It is known that the recording capacity of an optical recording medium can be increased by using light of a shorter wavelength and by increasing the numerical aperture (NA) of an objective lens. Concerning a shorter wavelength, the development of a semiconductor laser with a shorter wavelength using a GaN substrate (for example, a semiconductor laser that emits a laser beam of 408 nm wavelength) has advanced to the point where this wavelength is becoming practical.
With the development of short wavelength semiconductor lasers, research and development of AODs (Advanced Optical Disks), also known as HD-DVDs, that provide increased data storage capacity of a single layer on one side of an optical disk by using short wavelength light is also advancing. As the AOD standard, the numerical aperture and disk thickness are selected to be about the same as those of DVDs, as discussed previously, with the numerical aperture (NA) and disk substrate thickness for an AOD being set at 0.65 and 0.6 mm, respectively.
Furthermore, research and development of Blu-ray disk (BD) systems that use a shorter wavelength of disk illuminating light, similar to AOD systems, has progressed, and the standardized values of numerical aperture and disk thickness for these systems are completely different from the corresponding DVD and CD values, with a numerical aperture (NA) of 0.85 and a disk substrate thickness of 0.1 mm being standard. Unless otherwise indicated, hereinafter, AODs and Blu-ray disks collectively will be referred to as “AODs.”
Accordingly, the development of an optical pickup device that can be used for three different types of optical recording media, such as AODs, DVDs and CDs as described above, has been demanded and objective lenses for mounting in such devices have already been proposed. For example, objective lenses that includes a diffractive surface on at least one objective lens surface are described in Japanese Laid-Open Patent Application 2001-195769. The objective lens described in this publication is designed to improve chromatic aberration generated in a single element objective lens and the spherical aberration that accompanies the difference in thickness of the substrates of each optical recording medium by using the diffracted light of a specified order from the diffractive surface corresponding to each of the optical recording media, such as the next generation of high density optical disks that may use, for example, light of a wavelength of 400 nm, as well as using diffracted light of a wavelength used with a DVD and diffracted light of a wavelength used with a CD.
In addition, the objective lens of Japanese Laid-Open Patent Application 2001-195769 is constructed so that a divergent light beam is incident on the diffractive surface when information is recorded on or reproduced from a CD, and the deterioration in tracking accuracy that occurs in conjunction with the deterioration of optical performance off the optical axis is controlled by using a collimated light beam incident on the diffractive surface when information is recorded on or reproduced from a DVD and when information is recorded on or reproduced from the next generation of high density optical disks, which may use light of a wavelength of 400 nm, where the numerical aperture NA is large.
In optical pickup devices using objective lenses as described above, the objective lens moves according to the tracking control and also moves in order to provide focusing. However, focusing and tracking efficiency may be impaired based on the mass of the objective lens and/or by the objective lens colliding with the optical recording medium, based on the varying working distance of the objective lens from the optical recording medium.
Avoiding problems in such tracking control and focusing is an extremely important issue, especially when recording and reproducing information with a plurality of optical recording media that use light beams of different wavelengths, that use different diffraction orders of different light beams, and that use different numerical apertures.